Radio communication systems employing the Institute of Electrical and Electronics Engineers (IEEE) 802.16e known as the Worldwide Interoperability for Microwave Access (WiMAX) that uses the Orthogonal Frequency Division Multiple Access (OFDMA) as a radio communication technique have been developed.
FIG. 14 is a diagram illustrating a WiMAX radio communication system. In a WiMAX radio access network 100, base stations 101 to 103 and a gateway 104 are connected. Radio communication is performed between the base stations 101 to 103 and mobile stations 31 and 32. The gateway 104 is a node used to connect networks compliant with different protocols, and is disposed in an upper-level carrier building or the like.
In the above-described radio communication system, a base station receives a frame in which radio waves from a plurality of mobile stations are mixed. Accordingly, in order to prevent the saturation of a receiving level and the lack of a noise margin in the base station, it is required that the levels of radio waves from the mobile stations be substantially the same. The mobile stations perform transmission power control.
On the other hand, the base station performs access control processing for individually assigning communication slots, which are arranged in a time direction (symbol direction) and a frequency direction (subcarrier frequency direction), to the mobile stations. As a result, communication is performed between the base station and each of the mobile stations.
When a mobile station has no communication slot, the mobile station transmits a Code Division Multiple Access (CDMA) Ranging code (hereinafter merely referred to as a Ranging) defined in the WiMAX protocol specification (IEEE 802.16e or the subsequent standard) to a base station with a dedicated transmission slot. The base station receives the Ranging as a communication access request from the mobile station.
As a transmission power control technique in the related art, a technique for determining the power of transmission of an uplink frame from a subscriber terminal to a base station is disclosed in Japanese Unexamined Patent Application Publication No. 2007-519327. A technique for detecting the level value of a received signal on the basis of a Ranging transmitted from a mobile terminal, comparing the detected level value with a control target value, and generating a control command used to adjust the transmission output of the mobile terminal on the basis of a result of the comparison is disclosed in Japanese Unexamined Patent Application Publication No. 2009-71585.